Compact grating displacement measurement system
with a 3 × 3 coupler
Chunhua Wei (魏春华), Shuhua Yan (颜树华)*, Cunbao Lin (林存宝),
Zhiguang Du (杜志广), and Guochao Wang (王国超)
College of Mechatronic Engineering and Automation, National University of Defense Technology,
Changsha 410073, China
*Corresponding author: yanshuhua996@163.com
Received December 31, 2014; accepted March 16, 2015; posted online April 8, 2015
We present a compact displacement measurement system possessing the capability of nanometer-scale precision.
On basis of integrating single grating with 3 × 3 coupler for phase shift in interference signal, the present scheme
features advantages of simple structure, convenient alignment, and insensitivity to air turbulence. Linear com-
parisons between our system and HP5530 show a residual error less than 81 nm during step motions along a
10 mm round-trip, and a discrepancy less than 15 nm in the case of 200 μm movement. We also demonstrate a
measurement stability test in a duration of 300 s, which shows the proposed scheme potentially performs better
than HP5530 in terms of long-term stability.
OCIS codes: 050.1950, 060.2430, 060.2920.
doi: 10.3788/COL201513.051301.
Displacement measurement by optical interferometry has
been an essential tool for nano-scale demand applications
in industry and scientific research
[1]
. Up to date, although
conventional la ser interferometers working on heterodyne
or homodyne principle are extensively used to measure the
displacement with sub-nanometer resolution, the cost of
that measurement system is still high, and the measure-
ment accuracy depends on not only the stability of the
laser wavelength, but also the compensation of air refrac-
tive index sensitive to the environment is tough to be pre-
cisely determined
[2,3]
. As another technology competent
to realize displacement measurement with nanometers
precision, grating interferometry provides measurement
benchmark with a real object, and thus has a relatively
low requirement for environmental conditions
[4]
. Resulting
from the merits of low cost, simple configuration, and easy
instrumentation, displacement measurement by grating
interferometry attracts more and more attention in
fields of biopharmaceuticals, ultra-precision machining,
materials science, and so on.
Classical grating displacement measurement systems,
such as the dual-grating measurement system, work on
principles of Moiré fringe in a long range but with low
resolution
[5,6]
. Recently, various types of grating interfer-
ometers have been developed to measure linear displace-
ment or multi-degree-of-freedom (DOF) position with
nanometer resolution, such as single-metric grating
measurement system
[7,8]
, Lit trow-type grating interferom-
eters
[9,10]
, heterodyne grating interferometers
[11,12]
, and
DOF linear encoders
[13]
. Although most of these systems
have their own merits and specific applications, they are
usually built by relatively complicated optical setup,
leading to difficulty in alignment. In this work, we focus
on developing a compact and transportable grating inter-
ferometer for linear displace ment measurement, which
innovatively integrates the grating interferometry with
a 3 × 3 coupler to construct a compact displacement mea-
surement system. In this scheme, the optical path is mini-
mized and DC noise of interference signals is suppressed,
so the anti-jamming ability, measurement stability, and
insensitivity to ambient change are largely enhanced,
making it capable for precision measurement and poten-
tial for low-budget instrumentation.
The schematic configuration of the whole system is
shown in Fig.
1. The single-wavelength laser beam emitted
from the He–Ne laser is coupled into a single-mod e fiber
for possibility of distant delivery. The coupled beam from
the other end of the single-mode fiber is collimated with a
collimator, and then perpendicularly illuminates the sur-
face of the reflection -type grating. After grating diffrac-
tion, the diffracted beams of the positive first order and
negative first order reflect in the same diffraction angle.
To acquire an interference signal, the symmetric diffracted
beams of plus or minus first orders (denoted with P, Q) are
coupled into the 3 × 3 coupler, and then optical interfer-
ence signals with phase shifts are generated from the three
outputs of coupler and detected by photo-detectors (PDs).
Fig. 1. Schematic diagram of the system configuration. C,
collimator.
COL 13(5), 051301(2015) CHINESE OPTICS LETTERS May 10, 2015
1671-7694/2015/051301(5) 051301-1 © 2015 Chinese Optics Letters